1. Technical Field
The present invention relates to a surface acoustic wave element, and particularly relates to a multiplex-mode filter having a wide pass band.
2. Related Art
A surface acoustic wave (SAW) element includes a SAW filter used as a frequency selective element in transmission circuits, a SAW resonator which outputs a constant resonant frequency and the like. The SAW filter is utilized as an intermediate frequency (IF) filter, a radio frequency filter, or the like in mobile communications terminals, such as a cellular phone, for example. Particularly, a lateral multiplex-mode SAW filter is widely utilized as the IF filter among SAW filters.
Incidentally, because communications systems in recent years adopted a digital communication system, an IF filter having a relatively wide pass band is needed. In FIG. 3, a schematic plan view of a SAW filter related to the conventional art is shown. A SAW filter 1 as shown in FIG. 3 has a configuration in which two SAW resonators 3 are arranged closely on a piezo-electric substrate 2, in parallel with the propagation direction of a surface acoustic wave. Each SAW resonator 3 comprises an interdigital transducer (IDT) 4, a reflector 5 and the like, and the above IDT4 has a configuration which includes a plurality of power-supply conductors 6 (6a, 6b) along the propagation direction of the surface acoustic wave, and in which an electrode finger 7a is extend-prepared from the central power-supply conductor 6a towards the power-supply conductor 6b at both sides, and at the same time an electrode finger 7b is extend-prepared from the power-supply conductor 6b at both sides towards the central power-supply conductor 6a. With this configuration, the pass band width is made wider and miniaturization is attained.
International Publication No. 00/67374 is an example of the related art.
In the SAW filter described above, regarding a width D of the power-supply conductor and the bandwidth, there is a relationship shown in FIG. 4. This relationship shows that the narrower the width D of the power-supply conductor, the wider the bandwidth. Accordingly, in order to widen the pass band of the filter, it is necessary to narrow the width D of the power-supply conductor.
In FIG. 5, a relationship between the width D and the resistance of the power-supply conductor is shown. This relationship indicates that there is an inversely proportional relationship between the width and the resistance portion of the power-supply conductor, and that if the width D of the power-supply conductor is narrowed, the resistance produced in the power-supply conductor becomes large. Also, in FIG. 6, a relationship between the resistance portion and a filter loss is shown. This relationship indicates that the filter loss increase, as the resistance portion produced in the power-supply conductor increases. Accordingly, if the width D of the power-supply conductor is narrowed in order to make the pass band of a filter wider, the resistance loss of the power-supply conductor will increase, and furthermore the insertion loss of the filter will increase, consequently.
Moreover, as for the power-supply conductor in the central portion, the width D of the power supply conductor is narrow, and the power supply conductor is routed from one end to the other end of the IDT along the propagation direction of the surface acoustic wave, and at the same time this power-supply conductor is coupled with an input terminal via a coupling conductor. Therefore, the resistance loss of the power-supply conductor will increase further, and consequently, the insertion loss of the filter will also increase further.